When should Christians fight to protect?

A former marine, Daniel Penny, was recently acquitted of all charges - despite putting a black homeless man, Jordan Neely, in a chokehold and killing him while they and others were travelling on a New York underground train.

Neely, apparently well-known locally for his Michael Jackson impressions, had allegedly made violent threats against other travellers on the subway car (as carriages are known). But he reportedly says he could not have lived with his conscience if any harm had come to his fellow passengers.

The details of the case can be read on their own, and the arguments, but in our increasingly violent societies it does raise the question as to when it is right for Christians to act to protect. We are familiar with state-sanctioned protective violence, via Romans 13, that is apportioned to state actors including the police and the military. But an off-duty cop? A retired Marine?